Shock Breakout

30 March, 2016


Here you can see the brilliant flash of a supernova as its core blasts through its surface. This is an animated cartoon made by NASA based on observations of a red supergiant star that exploded in 2011. It has been sped up by a factor of 240. You can see a graph of brightness showing the actual timescale at lower right.

When a star like this runs out of fuel for nuclear fusion, its core cools. That makes the pressure drop—so the core collapses under the force of gravity.

When the core of a supernova collapses, the infalling matter can reach almost a quarter the speed of light. So when it hits the center, this matter becomes very hot! Indeed, the temperature can reach 100 billion kelvin. That’s 6000 times the temperature of our Sun’s core!

For a supernova less than 25 solar masses, the collapse stops only when the core is compressed into a neutron star. As this happens, lots of electrons and protons become neutrons and neutrinos. Most of the resulting energy is instantly carried away by a ten-second burst of neutrinos. This burst can have an energy of 1046 joules.

It’s hard to comprehend this. It’s what you’d get if you suddenly converted the mass of 18,000 Earths into energy! Astronomers use a specially huge unit with such energies: the foe, which stands for ten to the fifty-one ergs.

That’s 1044 joules. So, a supernova can release 100 foe in neutrinos. By comparison, only 1 or 2 foe come out as light.

Why? Neutrinos can effortlessly breeze through matter. Light cannot! So it takes longer to actually see things happen at the star’s surface—especially since a red supergiant is large. This one was about 500 times the radius of our Sun.

So what happened? A shock wave rushed upward through the star. First it broke through the star’s surface in the form of finger-like plasma jets, which you can see in the animation.

20 minutes later, the full fury of the shock wave reached the surface—and the doomed star exploded in a blinding flash! This is called the shock breakout.

Then the star expanded as a blue-hot ball of plasma.

Here’s how the star’s luminosity changed with time, measured in multiples of the Sun’s luminosity:



Note that while the shock breakout seems very bright, it’s ultimately dwarfed by the luminosity of the expanding ball of plasma. So, KSN2011d was actually one of the first two supernovae for which the shock breakout was seen! For details, read this:

• P. M. Garnavich, B. E. Tucker, A. Rest, E. J. Shaya, R. P. Olling, D. Kasen and A. Villar, Shock breakout and early light curves of Type II-P supernovae observed with Kepler.

A Type II supernova is one that shows hydrogen in its spectral lines: these are commonly formed by the collapse of a star that has run out of fuel in its core, but retains hydrogen in its outer layers. A Type II-P is one that shows a plateau in its light curve: the P is for ‘plateau’. These are more common than the Type II-L, which show a more rapid (‘linear’) decay in their luminosity:




Hard X-Ray Burst

25 February, 2016

I just learned something cool: 0.4 seconds after LIGO saw those gravitational waves on 14 September 2015, a satellite named Fermi detected a burst of X-rays!

• V. Connaughton et al, Fermi GBM observations of LIGO gravitational wave event GW150914.

It lasted one second. It was rather weak (for such things). The photons emitted ranged from 50 keV to 10 MeV in energy, with a peak around 3.5 MeV. The paper calls this event a ‘hard X-ray source’. Wikipedia says photons with an energy over 100 keV deserve the name gamma rays, while those between 10 keV and 100 keV are ‘hard X-rays’. So, maybe this event deserves to be maybe a gamma ray burst. I suppose it’s all just a matter of semantics: it’s not as if there’s any sharp difference between a highly energetic X-ray and a low-energy gamma ray.

Whatever you call it, this event does not appear connected with other previously known objects. It’s hard to tell exactly where it happened. But its location is consistent with what little we know about the source of the gravitational waves.

If this X-ray burst was caused by the same event that created the gravitational waves, that would be surprising. Everyone assumed the gravitational waves were formed by two large black holes that had been orbiting each other for millions or billions of years, slowly spiraling down. In this scenario we don’t expect much electromagnetic radiation when the black holes finally collide.

Perhaps those expectations are wrong. Or maybe—just maybe—both the gravitational waves and X-rays were formed during the collapse of a single very large star! That’s what typically causes gamma ray bursts—we think. But it’s not at all typical—as far as we know—for a large star to form two black holes when it collapses! And that’s what we’d need to get that gravitational wave event: two black holes, which then spiral down and merge into one!

Here’s an analysis of the issue:

• Abraham Loeb, Electromagnetic counterparts to black hole mergers detected by LIGO.

As he notes, the collapsing star would need to have an insane amount of angular momentum to collapse into a dumb-bell shape and form two black holes, each roughly 30 times the mass of our Sun, which then quickly spiral down and collide.

Furthermore, as Tony Wells pointed to me, the lack of neutrinos argues against the idea that this event involved a large collapsing star:

• ANTARES collaboration, High-energy neutrino follow-up search of Gravitational wave event GW150914 with ANTARES and IceCube.

To add to the muddle, another satellite devoted to observing gamma rays, called INTEGRAL, did not see anything:

• V. Savchenko et al, INTEGRAL upper limits on gamma-ray emission associated with the gravitational wave event GW150914.

It will take a while to sort this out.

But luckily, the first gravitational wave burst seen by LIGO was not the only one! Dennis Overbye of the New York Times writes:

Shortly after the September event, LIGO recorded another, weaker signal that was probably also from black holes, the team said. According to Dr. Weiss, there were at least four detections during the first LIGO observing run, which ended in January. The second run will begin this summer. In the fall, another detector, Advanced Virgo, operated by the European Gravitational Observatory in Italy, will start up. There are hopes for more in the future, in India and Japan.

So we will know more soon!

For more on Fermi:


Rumors of Gravitational Waves

7 February, 2016

The Laser Interferometric Gravitational-Wave Observatory or LIGO is designed to detect gravitational waves—ripples of curvature in spacetime moving at the speed of light. It’s recently been upgraded, and it will either find gravitational waves soon or something really strange is going on.

Rumors are swirling that LIGO has seen gravitational waves produced by two black holes, of 29 and 36 solar masses, spiralling towards each other—and then colliding to form a single 62-solar-mass black hole!

You’ll notice that 29 + 36 is more than 62. So, it’s possible that three solar masses were turned into energy, mostly in the form of gravitational waves!

According to these rumors, the statistical significance of the signal is supposedly very high: better than 5 sigma! That means there’s at most a 0.000057% probability this event is a random fluke – assuming nobody made a mistake.

If these rumors are correct, we should soon see an official announcement. If the discovery holds up, someone will win a Nobel prize.

The discovery of gravitational waves is completely unsurprising, since they’re predicted by general relativity, a theory that’s passed many tests already. But it would open up a new window to the universe – and we’re likely to see interesting new things, once gravitational wave astronomy becomes a thing.

Here’s the tweet that launched the latest round of rumors:

ligo_tweet_cliff_burgess

For background on this story, try this:

Tale of a doomed galaxy, Azimuth, 8 November 2015.

The first four sections of that long post discuss gravitational waves created by black hole collisions—but the last section is about LIGO and an earlier round of rumors, so I’ll quote it here!


LIGO stands for Laser Interferometer Gravitational Wave Observatory. The idea is simple. You shine a laser beam down two very long tubes and let it bounce back and forth between mirrors at the ends. You use this compare the length of these tubes. When a gravitational wave comes by, it stretches space in one direction and squashes it in another direction. So, we can detect it.

Sounds easy, eh? Not when you run the numbers! We’re trying to see gravitational waves that stretch space just a tiny bit: about one part in 1023. At LIGO, the tubes are 4 kilometers long. So, we need to see their length change by an absurdly small amount: one-thousandth the diameter of a proton!

It’s amazing to me that people can even contemplate doing this, much less succeed. They use lots of tricks:

• They bounce the light back and forth many times, effectively increasing the length of the tubes to 1800 kilometers.

• There’s no air in the tubes—just a very good vacuum.

• They hang the mirrors on quartz fibers, making each mirror part of a pendulum with very little friction. This means it vibrates very well at one particular frequency, and very badly at frequencies far from that. This damps out the shaking of the ground, which is a real problem.

• This pendulum is hung on another pendulum.

• That pendulum is hung on a third pendulum.

• That pendulum is hung on a fourth pendulum.

• The whole chain of pendulums is sitting on a device that detects vibrations and moves in a way to counteract them, sort of like noise-cancelling headphones.

• There are 2 of these facilities, one in Livingston, Louisiana and another in Hanford, Washington. Only if both detect a gravitational wave do we get excited.

I visited the LIGO facility in Louisiana in 2006. It was really cool! Back then, the sensitivity was good enough to see collisions of black holes and neutron stars up to 50 million light years away.

Here I’m not talking about the supermassive black holes that live in the centers of galaxies. I’m talking about the much more common black holes and neutron stars that form when stars go supernova. Sometimes a pair of stars orbiting each other will both blow up, and form two black holes—or two neutron stars, or a black hole and neutron star. And eventually these will spiral into each other and emit lots of gravitational waves right before they collide.

50 million light years is big enough that LIGO could see about half the galaxies in the Virgo Cluster. Unfortunately, with that many galaxies, we only expect to see one neutron star collision every 50 years or so.

They never saw anything. So they kept improving the machines, and now we’ve got Advanced LIGO! This should now be able to see collisions up to 225 million light years away… and after a while, three times further.

They turned it on September 18th. Soon we should see more than one gravitational wave burst each year.

In fact, there’s a rumor that they’ve already seen one! But they’re still testing the device, and there’s a team whose job is to inject fake signals, just to see if they’re detected. Davide Castelvecchi writes:

LIGO is almost unique among physics experiments in practising ‘blind injection’. A team of three collaboration members has the ability to simulate a detection by using actuators to move the mirrors. “Only they know if, and when, a certain type of signal has been injected,” says Laura Cadonati, a physicist at the Georgia Institute of Technology in Atlanta who leads the Advanced LIGO’s data-analysis team.

Two such exercises took place during earlier science runs of LIGO, one in 2007 and one in 2010. Harry Collins, a sociologist of science at Cardiff University, UK, was there to document them (and has written books about it). He says that the exercises can be valuable for rehearsing the analysis techniques that will be needed when a real event occurs. But the practice can also be a drain on the team’s energies. “Analysing one of these events can be enormously time consuming,” he says. “At some point, it damages their home life.”

The original blind-injection exercises took 18 months and 6 months respectively. The first one was discarded, but in the second case, the collaboration wrote a paper and held a vote to decide whether they would make an announcement. Only then did the blind-injection team ‘open the envelope’ and reveal that the events had been staged.

Aargh! The disappointment would be crushing.

But with luck, Advanced LIGO will soon detect real gravitational waves. And I hope life here in the Milky Way thrives for a long time – so that when the gravitational waves from the doomed galaxy PG 1302-102 reach us, hundreds of thousands of years in the future, we can study them in exquisite detail.

For Castelvecchi’s whole story, see:

• Davide Castelvecchi Has giant LIGO experiment seen gravitational waves?, Nature, 30 September 2015.

For pictures of my visit to LIGO, see:

• John Baez, This week’s finds in mathematical physics (week 241), 20 November 2006.

For how Advanced LIGO works, see:

• The LIGO Scientific Collaboration Advanced LIGO, 17 November 2014.


Aggressively Expanding Civilizations

5 February, 2016

Ever since I became an environmentalist, the potential destruction wrought by aggressively expanding civilizations has been haunting my thoughts. Not just here and now, where it’s easy to see, but in the future.

In October 2006, I wrote this in my online diary:

A long time ago on this diary, I mentioned my friend Bruce Smith’s nightmare scenario. In the quest for ever faster growth, corporations evolve toward ever faster exploitation of natural resources. The Earth is not enough. So, ultimately, they send out self-replicating von Neumann probes that eat up solar systems as they go, turning the planets into more probes. Different brands of probes will compete among each other, evolving toward ever faster expansion. Eventually, the winners will form a wave expanding outwards at nearly the speed of light—demolishing everything behind them, leaving only wreckage.

The scary part is that even if we don’t let this happen, some other civilization might.

The last point is the key one. Even if something is unlikely, in a sufficiently large universe it will happen, as long as it’s possible. And then it will perpetuate itself, as long as it’s evolutionarily fit. Our universe seems pretty darn big. So, even if a given strategy is hard to find, if it’s a winning strategy it will get played somewhere.

So, even in this nightmare scenario of "spheres of von Neumann probes expanding at near lightspeed", we don’t need to worry about a bleak future for the universe as a whole—any more than we need to worry that viruses will completely kill off all higher life forms. Some fraction of civilizations will probably develop defenses in time to repel the onslaught of these expanding spheres.

It’s not something I stay awake worrying about, but it’s a depressingly plausible possibility. As you can see, I was trying to reassure myself that everything would be okay, or at least acceptable, in the long run.

Even earlier, S. Jay Olson and I wrote a paper together on the limitations in accurately measuring distances caused by quantum gravity. If you try to measure a distance too accurately, you’ll need to concentrate so much energy in such a small space that you’ll create a black hole!

That was in 2002. Later I lost touch with him. But now I’m happy to discover that he’s doing interesting work on quantum gravity and quantum information processing! He is now at Boise State University in Idaho, his home state.

But here’s the cool part: he’s also studying aggressively expanding civilizations.

Expanding bubbles

What will happen if some civilizations start aggressively expanding through the Universe at a reasonable fraction of the speed of light? We don’t have to assume most of them do. Indeed, there can’t be too many, or they’d already be here! More precisely, the density of such civilizations must be low at the present time. The number of them could be infinite, since space is apparently infinite. But none have reached us. We may eventually become such a civilization, but we’re not one yet.

Each such civilization will form a growing ‘bubble’: an expanding sphere of influence. And occasionally, these bubbles will collide!

Here are some pictures from a simulation he did:





As he notes, the math of these bubbles has already been studied by researchers interested in inflationary cosmology, like Alan Guth. These folks have considered the possibility that in the very early Universe, most of space was filled with a ‘false vacuum’: a state of matter that resembles the actual vacuum, but has higher energy density.

A false vacuum could turn into the true vacuum, liberating energy in the form of particle-antiparticle pairs. However, it might not do this instantly! It might be ‘metastable’, like ball number 1 in this picture:

It might need a nudge to ‘roll over the hill’ (metaphorically) and down into the lower-energy state corresponding to the true vacuum, shown as ball number 3. Or, thanks to quantum mechanics, it might ‘tunnel’ through this hill.

The balls and the hill are just an analogy. What I mean is that the false vacuum might need to go through a stage of having even higher energy density before it could turn into the true vacuum. Random fluctuations, either quantum-mechanical or thermal, could make this happen. Such a random fluctuation could happen in one location, forming a ‘bubble’ of true vacuum that—under certain conditions—would rapidly expand.

It’s actually not very different from bubbles of steam forming in superheated water!

But here’s the really interesting Jay Olson noted in his first paper on this subject. Research on bubbles in the inflationary cosmology could actually be relevant to aggressively expanding civilizations!

Why? Just as a bubble of expanding true vacuum has different pressure than the false vacuum surrounding it, the same might be true for an aggressively expanding civilization. If they are serious about expanding rapidly, they may convert a lot of matter into radiation to power their expansion. And while energy is conserved in this process, the pressure of radiation in space is a lot bigger than the pressure of matter, which is almost zero.

General relativity says that energy density slows the expansion of the Universe. But also—and this is probably less well-known among nonphysicists—it says that pressure has a similar effect. Also, as the Universe expands, the energy density and pressure of radiation drops at a different rate than the energy density of matter.

So, the expansion of the Universe itself, on a very large scale, could be affected by aggressively expanding civilizations!

The fun part is that Jay Olson actually studies this in a quantitative way, making some guesses about the numbers involved. Of course there’s a huge amount of uncertainty in all matters concerning aggressively expanding high-tech civilizations, so he actually considers a wide range of possible numbers. But if we assume a civilization turns a large fraction of matter into radiation, the effects could be significant!

The effect of the extra pressure due to radiation would be to temporarily slow the expansion of the Universe. But the expansion would not be stopped. The radiation will gradually thin out. So eventually, dark energy—which has negative pressure, and does not thin out as the Universe expands—will win. Then the Universe will expand exponentially, as it is already beginning to do now.

(Here I am ignoring speculative theories where dark energy has properties that change dramatically over time.)

Jay Olson’s work

Here are his papers on this subject. The abstracts sketch his results, but you have to look at the papers to see how nice they are. He’s thought quite carefully about these things.

• S. Jay Olson, Homogeneous cosmology with aggressively expanding civilizations, Classical and Quantum Gravity 32 (2015) 215025.

Abstract. In the context of a homogeneous universe, we note that the appearance of aggressively expanding advanced life is geometrically similar to the process of nucleation and bubble growth in a first-order cosmological phase transition. We exploit this similarity to describe the dynamics of life saturating the universe on a cosmic scale, adapting the phase transition model to incorporate probability distributions of expansion and resource consumption strategies. Through a series of numerical solutions spanning several orders of magnitude in the input assumption parameters, the resulting cosmological model is used to address basic questions related to the intergalactic spreading of life, dealing with issues such as timescales, observability, competition between strategies, and first-mover advantage. Finally, we examine physical effects on the universe itself, such as reheating and the backreaction on the evolution of the scale factor, if such life is able to control and convert a significant fraction of the available pressureless matter into radiation. We conclude that the existence of life, if certain advanced technologies are practical, could have a significant influence on the future large-scale evolution of the universe.

• S. Jay Olson, Estimates for the number of visible galaxy-spanning civilizations and the cosmological expansion of life.

Abstract. If advanced civilizations appear in the universe with a desire to expand, the entire universe can become saturated with life on a short timescale, even if such expanders appear but rarely. Our presence in an untouched Milky Way thus constrains the appearance rate of galaxy-spanning Kardashev type III (K3) civilizations, if it is assumed that some fraction of K3 civilizations will continue their expansion at intergalactic distances. We use this constraint to estimate the appearance rate of K3 civilizations for 81 cosmological scenarios by specifying the extent to which humanity could be a statistical outlier. We find that in nearly all plausible scenarios, the distance to the nearest visible K3 is cosmological. In searches where the observable range is limited, we also find that the most likely detections tend to be expanding civilizations who have entered the observable range from farther away. An observation of K3 clusters is thus more likely than isolated K3 galaxies.

• S. Jay Olson, On the visible size and geometry of aggressively expanding civilizations at cosmological distances.

Abstract. If a subset of advanced civilizations in the universe choose to rapidly expand into unoccupied space, these civilizations would have the opportunity to grow to a cosmological scale over the course of billions of years. If such life also makes observable changes to the galaxies they inhabit, then it is possible that vast domains of life-saturated galaxies could be visible from the Earth. Here, we describe the shape and angular size of these domains as viewed from the Earth, and calculate median visible sizes for a variety of scenarios. We also calculate the total fraction of the sky that should be covered by at least one domain. In each of the 27 scenarios we examine, the median angular size of the nearest domain is within an order of magnitude of a percent of the whole celestial sphere. Observing such a domain would likely require an analysis of galaxies on the order of a giga-lightyear from the Earth.

Here are the main assumptions in his first paper:

1. At early times (relative to the appearance of life), the universe is described by the standard cosmology – a benchmark Friedmann-Robertson-Walker (FRW) solution.

2. The limits of technology will allow for self-reproducing spacecraft, sustained relativistic travel over cosmological distances, and an efficient process to convert baryonic matter into radiation.

3. Control of resources in the universe will tend to be dominated by civilizations that adopt a strategy of aggressive expansion (defined as a frontier which expands at a large fraction of the speed of the individual spacecraft involved), rather than those expanding diffusively due to the conventional pressures of population dynamics.

4. The appearance of aggressively expanding life in the universe is a spatially random event and occurs at some specified, model-dependent rate.

5. Aggressive expanders will tend to expand in all directions unless constrained by the presence of other civilizations, will attempt to gain control of as much matter as is locally available for their use, and once established in a region of space, will consume mass as an energy source (converting it to radiation) at some specified, model-dependent rate.


Curiosity Meets Martian Dunes

17 January, 2016

In December, the rover Curiosity reached some sand dunes on Mars, giving us the first views of these dunes taken from the ground instead of from above. It’s impressive how the dune seems to shoot straight up from the rocks here!

In fact this slope—the steep downwind slope of one of “Bagnold Dunes” along the northwestern flank of Mount Sharp—is just about 27°. But mountaineers will confirm that slopes always looks steeper than they are.

The wind makes this dune move about one meter per year.

For more, see:

• NASA, NASA Mars rover Curiosity reaches sand dunes, 10 December 2015.

• Jet Propulsion Laboratory, Mastcam telephoto of a Martian dune’s downwind face, 4 January 2016.

• Jet Propulsion Laboratory, Slip face on downwind side of ‘Namib’ sand dune on Mars, 6 January 2016.



Tale of a Doomed Galaxy

8 November, 2015

Part 1



About 3 billion years ago, if there was intelligent life on the galaxy we call PG 1302-102, it should have known it was in serious trouble.

Our galaxy has a supermassive black hole in the middle. But that galaxy had two. One was about ten times as big as the other. Taken together, they weighed a billion times as much as our Sun.

They gradually spiraled in towards each other… and then, suddenly, one fine morning, they collided. The resulting explosion was 10 million times more powerful than a supernova—more powerful than anything astronomers here on Earth have ever seen! It was probably enough to wipe out all life in that galaxy.

We haven’t actually seen this yet. The light and gravitational waves from the disaster are still speeding towards us. They should reach us in roughly 100,000 years. We’re not sure when.

Right now, we see the smaller black hole still orbiting the big one, once every 5 years. In fact it’s orbiting once every 4 years! But thanks to the expansion of the universe, PG 1302-102 is moving away from us so fast that time on that distant galaxy looks significantly slowed down to us.

Orbiting once every 4 years: that doesn’t sound so fast. But the smaller black hole is about 2000 times more distant from its more massive companion than Pluto is from our Sun! So in fact it’s moving at very high speed – about 1% of the speed of light. We can actually see it getting redshifted and then blueshifted as it zips around. And it will continue to speed up as it spirals in.

What exactly will happen when these black holes collide? It’s too bad we won’t live to see it. We’re far enough that it will be perfectly safe to watch from here! But the human race knows enough about physics to say quite a lot about what it will be like. And we’ve built some amazing machines to detect the gravitational waves created by collisions like this—so as time goes on, we’ll know even more.

Part 2



Even before the black holes at the heart of PG 1302-102 collided, life in that galaxy would have had a quasar to contend with!

This is a picture of Centaurus A, a much closer galaxy with a quasar in it. A quasar is huge black hole in the middle of a galaxy—a black hole that’s eating lots of stars, which rip apart and form a disk of hot gas as they spiral in. ‘Hot’ is an understatement, since this gas moves near the speed of light. It gets so hot that it pumps out intense jets of particles – from its north and south poles. Some of these particles even make it to Earth.

Any solar system in Centaurus A that gets in the way of those jets is toast.

And these jets create lots of radiation, from radio waves to X-rays. That’s how we can see quasars from billions of light years away. Quasars are the brightest objects in the universe, except for short-lived catastrophic events like the black hole collisions and gamma-ray bursts from huge dying stars.

It’s hard to grasp the size and power of such things, but let’s try. You can’t see the black hole in the middle of this picture, but it weighs 55 million times as much as our Sun. The blue glow of the jets in this picture is actually X rays. The jet at upper left is 13,000 light years long, made of particles moving at half the speed of light.

A typical quasar puts out a power of roughly 1040 watts. They vary a lot, but let’s pick this number as our ‘standard quasar’.

But what does 1040 watts actually mean? For comparison, the Sun puts out 4 x 1026 watts. So, we’re talking 30 trillion Suns. But even that’s too big a number to comprehend!

Maybe it would help to say that the whole Milky Way puts out 5 x 1036 watts. So a single quasar, at the center of one galaxy, can have the power of 2000 galaxies like ours.

Or, we can work out how much energy would be produced if the entire mass of the Moon were converted into energy. I’m getting 6 x 1039 joules. That’s a lot! But our standard quasar is putting out a bit more power than if it were converting one Moon into energy each second.

But you can’t just turn matter completely into energy: you need an equal amount of antimatter, and there’s not that much around. A quasar gets its power the old-fashioned way: by letting things fall down. In this case, fall down into a black hole.

To power our standard quasar, 10 stars need to fall into the black hole every year. The biggest quasars eat 1000 stars a year. The black hole in our galaxy gets very little to eat, so we don’t have a quasar.

There are short-lived events much more powerful than a quasar. For example, a gamma-ray burst, formed as a hypergiant star collapses into a black hole. A powerful gamma-ray burst can put out 10^44 watts for a few seconds. That’s equal to 10,000 quasars! But quasars last a long, long time.

So this was life in PG 1302-102 before things got really intense – before its two black holes spiraled into each other and collided. What was that collision like? I’ll talk about that next time.

The above picture of Centaurus A was actually made from images taken by three separate telescopes. The orange glow is submillimeter radiation – between infrared and microwaves—detected by the Atacama Pathfinder Experiment (APEX) telescope in Chile. The blue glow is X-rays seen by the Chandra X-ray Observatory. The rest is a photo taken in visible light by the Wide Field Imager on the Max-Planck/ESO 2.2 meter telescope, also located in Chile. This shows the dust lanes in the galaxy and background stars.

Part 3


What happened at the instant the supermassive black holes in the galaxy PG 1302-102 finally collided?

We’re not sure yet, because the light and gravitational waves will take time to get here. But physicists are using computers to figure out what happens when black hole collide!

Here you see some results. The red blobs are the event horizons of two black holes.

First the black holes orbit each other, closer and closer, as they lose energy by emitting gravitational radiation. This is called the ‘inspiral’ phase.

Then comes the ‘plunge’ and ‘merger’. They plunge towards each other. A thin bridge forms between them, which you see here. Then they completely merge.

Finally you get a single black hole, which oscillates and then calms down. This is called the ‘ringdown’, because it’s like a bell ringing, loudly at first and then more quietly. But instead of emitting sound, it’s emitting gravitational waves—ripples in the shape of space!

In the top picture, the black holes have the same mass: one looks smaller, but that’s because it’s farther away. In the bottom picture, the black hole at left is twice as massive.

Here’s one cool discovery. An earlier paper had argued there could be two bridges, except in very symmetrical situations. If that were true, a black hole could have the topology of a torus for a little while. But these calculations showed that – at least in the cases they looked at—there’s just one bridge.

So, you can’t have black hole doughnuts. At least not yet.

These calculations were done using free software called SpEC. But before you try to run it at home: the team that puts out this software says:

Because of the steep learning curve and complexity of SpEC, new users are typically introduced to SpEC through a collaboration with experienced SpEC users.

It probably requires a lot of computer power, too. These calculations are very hard. We know the equations; they’re just tough to solve. The first complete simulation of an inspiral, merger and ringdown was done in 2005.

The reason people want to simulate colliding black holes is not mainly to create pretty pictures, or even understand what happens to the event horizon. It’s to understand the gravitational waves they will produce! People are building better and better gravitational wave detectors—more on that later—but we still haven’t seen gravitational waves. This is not surprising: they’re very weak. To find them, we need to filter out noise. So, we need to know what to look for.

The pictures are from here:

• Michael I. Cohen and Jeffrey D. Kaplan and Mark A. Scheel, On toroidal horizons in binary black hole inspirals, Phys. Rev. D 85 (2012), 024031.

Part 4

Let’s imagine an old, advanced civilization in the doomed galaxy PG 1302-102.

Long ago they had mastered space travel. Thus, they were able to survive when their galaxy collided with another—just as ours will collide with Andromeda four billion years from now. They had a lot of warning—and so do we. The picture here shows what Andromeda will look like 250 million years before it hits.

They knew everything we do about astronomy—and more. So they knew that when galaxies collide, almost all stars sail past each other unharmed. A few planets get knocked out of orbit. Colliding clouds of gas and dust form new stars, often blue giants that live short, dramatic lives, going supernova after just 10 million years.

All this could be handled by not being in the wrong place at the wrong time. They knew the real danger came from the sleeping monsters at the heart of the colliding galaxies.

Namely, the supermassive black holes!

Almost every galaxy has a huge black hole at its center. This black hole is quiet when not being fed. But when galaxies collide, lots of gas and dust and even stars get caught by the gravity and pulled in. This material form a huge flat disk as it spirals down and heats up. The result is an active galactic nucleus.

In the worst case, the central black holes can eat thousands of stars a year. Then we get a quasar, which easily pumps out the power of 2000 ordinary galaxies.

Much of this power comes out in huge jets of X-rays. These jets keep growing, eventually stretching for hundreds of thousands of light years. The whole galaxy becomes bathed in X-rays—killing all life that’s not prepared.

Let’s imagine a civilization that was prepared. Natural selection has ways of weeding out civilizations that are bad at long-term planning. If you’re prepared, and you have the right technology, a quasar could actually be a good source of power.

But the quasar was just the start of the problem. The combined galaxy had two black holes at its center. The big one was at least 400 million times the mass of our Sun. The smaller one was about a tenth as big—but still huge.

They eventually met and started to orbit each other. By flinging stars out the way, they gradually came closer. It was slow at first, but the closer they got, the faster they circled each other, and the more gravitational waves they pumped out. This carried away more energy—so they moved closer, and circled even faster, in a dance with an insane, deadly climax.

Right now—here on Earth, where it takes a long time for the news to reach us—we see that in 100,000 years the two black holes will spiral down completely, collide and merge. When this happens, a huge pulse of gravitational waves, electromagnetic radiation, matter and even antimatter will blast through the galaxy called PG 1302-102.

I don’t know exactly what this will be like. I haven’t found papers describing this kind of event in detail.

One expert told the New York Times that the energy of this explosion will equal 100 million supernovae. I don’t think he was exaggerating. A supernova is a giant star whose core collapses as it runs out of fuel, easily turning several Earth masses of hydrogen into iron before you can say “Jack Robinson”. When it does this, it can easily pump out 1044 joules of energy. So, 100 millon supernovae is 1052 joules. By contrast, if we could convert all the mass of the black holes in PG 1302-102. into energy, we’d get about 1056 joules. So, our expert was just saying that their merger will turns 0.01% of their combined mass into energy. That seems quite reasonable to me.

But I want to know what happens then! What will the explosion do to the galaxy? Most of the energy comes out as gravitational radiation. Gravitational waves don’t interact very strongly with matter. But when they’re this strong, who knows? And of course there will be plenty of ordinary radiation, as the accretion disk gets shredded and sucked into the new combined black hole.

The civilization I’m imagining was smart enough not to stick around. They decided to simply leave the galaxy.

After all, they could tell the disaster was coming, at least a million years in advance. Some may have decided to stay and rough it out, or die a noble death. But most left.

And then what?

It takes a long time to reach another galaxy. Right now, travelling at 1% the speed of light, it would take 250 million years to reach Andromeda from here.

But they wouldn’t have to go to another galaxy. They could just back off, wait for the fireworks to die down, and move back in.

So don’t feel bad for them. I imagine they’re doing fine.

By the way, the expert I mentioned is S. George Djorgovski of Caltech, mentioned here:

• Dennis Overbye, Black holes inch ahead to violent cosmic union, New York Times, 7 January 2015.

Part 5


When distant black holes collide, they emit a burst of gravitational radiation: a ripple in the shape of space, spreading out at the speed of light. Can we detect that here on Earth? We haven’t yet. But with luck we will soon, thanks to LIGO.

LIGO stands for Laser Interferometer Gravitational Wave Observatory. The idea is simple. You shine a laser beam down two very long tubes and let it bounce back and forth between mirrors at the ends. You use this compare the length of these tubes. When a gravitational wave comes by, it stretches space in one direction and squashes it in another direction. So, we can detect it.

Sounds easy, eh? Not when you run the numbers! We’re trying to see gravitational waves that stretch space just a tiny bit: about one part in 1023. At LIGO, the tubes are 4 kilometers long. So, we need to see their length change by an absurdly small amount: one-thousandth the diameter of a proton!

It’s amazing to me that people can even contemplate doing this, much less succeed. They use lots of tricks:

• They bounce the light back and forth many times, effectively increasing the length of the tubes to 1800 kilometers.

• There’s no air in the tubes—just a very good vacuum.

• They hang the mirrors on quartz fibers, making each mirror part of a pendulum with very little friction. This means it vibrates very well at one particular frequency, and very badly at frequencies far from that. This damps out the shaking of the ground, which is a real problem.

• This pendulum is hung on another pendulum.

• That pendulum is hung on a third pendulum.

• That pendulum is hung on a fourth pendulum.

• The whole chain of pendulums is sitting on a device that detects vibrations and moves in a way to counteract them, sort of like noise-cancelling headphones.

• There are 2 of these facilities, one in Livingston, Louisiana and another in Hanford, Washington. Only if both detect a gravitational wave do we get excited.

I visited the LIGO facility in Louisiana in 2006. It was really cool! Back then, the sensitivity was good enough to see collisions of black holes and neutron stars up to 50 million light years away.

Here I’m not talking about supermassive black holes like the ones in the doomed galaxy of my story here! I’m talking about the much more common black holes and neutron stars that form when stars go supernova. Sometimes a pair of stars orbiting each other will both blow up, and form two black holes—or two neutron stars, or a black hole and neutron star. And eventually these will spiral into each other and emit lots of gravitational waves right before they collide.

50 million light years is big enough that LIGO could see about half the galaxies in the Virgo Cluster. Unfortunately, with that many galaxies, we only expect to see one neutron star collision every 50 years or so.

They never saw anything. So they kept improving the machines, and now we’ve got Advanced LIGO! This should now be able to see collisions up to 225 million light years away… and after a while, three times further.

They turned it on September 18th. Soon we should see more than one gravitational wave burst each year.

In fact, there’s a rumor that they’ve already seen one! But they’re still testing the device, and there’s a team whose job is to inject fake signals, just to see if they’re detected. Davide Castelvecchi writes:

LIGO is almost unique among physics experiments in practising ‘blind injection’. A team of three collaboration members has the ability to simulate a detection by using actuators to move the mirrors. “Only they know if, and when, a certain type of signal has been injected,” says Laura Cadonati, a physicist at the Georgia Institute of Technology in Atlanta who leads the Advanced LIGO’s data-analysis team.

Two such exercises took place during earlier science runs of LIGO, one in 2007 and one in 2010. Harry Collins, a sociologist of science at Cardiff University, UK, was there to document them (and has written books about it). He says that the exercises can be valuable for rehearsing the analysis techniques that will be needed when a real event occurs. But the practice can also be a drain on the team’s energies. “Analysing one of these events can be enormously time consuming,” he says. “At some point, it damages their home life.”

The original blind-injection exercises took 18 months and 6 months respectively. The first one was discarded, but in the second case, the collaboration wrote a paper and held a vote to decide whether they would make an announcement. Only then did the blind-injection team ‘open the envelope’ and reveal that the events had been staged.

Aargh! The disappointment would be crushing.

But with luck, Advanced LIGO will soon detect real gravitational waves. And I hope life here in the Milky Way thrives for a long time – so that when the gravitational waves from the doomed galaxy PG 1302-102 reach us, hundreds of thousands of years in the future, we can study them in exquisite detail.

For Castelvecchi’s whole story, see:

• Davide Castelvecchi Has giant LIGO experiment seen gravitational waves?, Nature, 30 September 2015.

For pictures of my visit to LIGO, see:

• John Baez, This week’s finds in mathematical physics (week 241), 20 November 2006.

For how Advanced LIGO works, see:

• The LIGO Scientific Collaboration Advanced LIGO, 17 November 2014.

References

To see where the pictures are from, click on them. For more, try this:

• Ravi Mandalia, Black hole binary entangled by gravity progressing towards deadly merge.

The picture of Andromeda in the nighttime sky 3.75 billion years from now was made by NASA. You can see a whole series of these pictures here:

• NASA, NASA’s Hubble shows Milky Way is destined for head-on collision, 31 March 2012.

Let’s get ready! For starters, let’s deal with global warming.


Scholz’s Star

19 February, 2015

100,000 years ago, some of my ancestors came out of Africa and arrived in the Middle East. 50,000 years ago, some of them reached Asia. But between those dates, about 70,000 years ago, two stars passed through the outer reaches of the Solar System, where icy comets float in dark space!

One was a tiny red dwarf called Scholz’s star. It’s only 90 times as heavy as Jupiter. Right now it’s 20 light years from us, so faint that it was discovered only in 2013, by Ralf-Dieter Scholz—an expert on nearby stars, high-velocity stars, and dwarf stars.

The other was a brown dwarf: a star so small that it doesn’t produce energy by fusion. This one is only 65 times the mass of Jupiter, and it orbits its companion at a distance of 80 AU.

(An AU, or astronomical unit, is the distance between the Earth and the Sun.)

A team of scientists has just computed that while some of my ancestors were making their way to Asia, these stars passed about 0.8 light years from our Sun. That’s not very close. But it’s close enough to penetrate the large cloud of comets surrounding the Sun: the Oort cloud.



They say this event didn’t affect the comets very much. But if it shook some comets loose from the Oort cloud, they would take about 2 million years to get here! So, they won’t arrive for a long time.

At its closest approach, Scholz’s star would have had an apparent magnitude of about 11.4. This is a bit too faint to see, even with binoculars. So, don’t look for it myths and legends!

As usual, the paper that made this discovery is expensive in journals but free on the arXiv:

• Eric E. Mamajek, Scott A. Barenfeld, Valentin D. Ivanov, Alexei Y. Kniazev, Petri Vaisanen, Yuri Beletsky, Henri M. J. Boffin, The closest known flyby of a star to the Solar System.

It must be tough being a scientist named ‘Boffin’, especially in England! Here’s a nice account of how the discovery was made:

• University of Rochester, A close call of 0.8 light years, 16 February 2015.

The brown dwarf companion to Scholz’s star is a ‘class T’ star. What does that mean? It’s pretty interesting. Let’s look at an example just 7 light years from Earth!

Brown dwarfs

 

Thanks to some great new telescopes, astronomers have been learning about weather on brown dwarfs! It may look like this artist’s picture. (It may not.)

Luhman 16 is a pair of brown dwarfs orbiting each other just 7 light years from us. The smaller one, Luhman 16B, is half covered by huge clouds. These clouds are hot—1200 °C—so they’re probably made of sand, iron or salts. Some of them have been seen to disappear! Why? Maybe ‘rain’ is carrying this stuff further down into the star, where it melts.

So, we’re learning more about something cool: the ‘L/T transition’.

Brown dwarfs can’t fuse ordinary hydrogen, but a lot of them fuse the isotope of hydrogen called deuterium that people use in H-bombs—at least until this runs out. The atmosphere of a hot brown dwarf is similar to that of a sunspot: it contains molecular hydrogen, carbon monoxide and water vapor. This is called a class M brown dwarf.

But as they run out of fuel, they cool down. The cooler class L brown dwarfs have clouds! But the even cooler class T brown dwarfs do not. Why not?

This is the mystery we may be starting to understand: the clouds may rain down, with material moving deeper into the star! Luhman 16B is right near the L/T transition, and we seem to be watching how the clouds can disappear as a brown dwarf cools. (Its larger companion, Luhman 16A, is firmly in class L.)

Finally, as brown dwarfs cool below 300 °C, astronomers expect that ice clouds start to form: first water ice, and eventually ammonia ice. These are the class Y brown dwarfs. Wouldn’t that be neat to see? A star with icy clouds!

Could there be life on some of these stars?

Caroline Morley regularly blogs about astronomy. If you want to know more about weather on Luhman 16B, try this:

• Caroline Morley, Swirling, patchy clouds on a teenage brown dwarf, 28 February 2012.

She doesn’t like how people call brown dwarfs “failed stars”. I agree! It’s like calling a horse a “failed giraffe”.

For more, try:

Brown dwarfs, Scholarpedia.